The knee is formed where the thigh bone meets the shin bone. The knee includes three bones, the femur (thigh bone), tibia (shin bone) and patella (knee cap). The distal end of the femur sits on top of the tibia, and the patella is on the front of the femur.
The three bones in the knee form two joints. The first is the tibio-femoral joint, which relates to the joint where the femur meets tibia. The second joint is the patello-femoral joint, which relates to the joint where the patella meets the femur.
The joint elements that engage one another are covered by articular cartilage. Specifically, the distal end of the femur and the proximal end of the tibia are covered by articular cartilage, as is the posterior surface of the patella. The articular cartilage is a slippery substance that absorbs shock and allows the bone surfaces to slide against one another without damage to either surface.
If the articular cartilage becomes damaged, the contact surface in the joint will not slide as well. The damaged area becomes a weak point in the cartilage that is prone to continued degradation over time. Ultimately, the damage can lead to bone-to-bone contact during articulation of the joint, resulting in significant pain and potential damage to the bone surfaces.
If the damage to the articular cartilage is sufficient enough, a knee replacement procedure may be required. Depending on the extent of the damage, the procedure may operate on either all or only a portion of the knee. For instance, in a total knee replacement, the ends of both the femur and the tibia are covered by prosthetics, as well as a surface of the patella. In other instances the procedure may be limited to portions of one of the joints. For instance, in a patello-femoral procedure, the procedure is limited to the engagement surfaces between the femur and the patella.